In head examinations involving magnetic resonance imaging, computer-aided tomography and other such techniques, it is desirable to have a well-defined, reproducible coordinate system to record and/or compare the locations and sizes of lesions, tumors and other structures of interest. Though there are a number of known devices and techniques for potential application to these types of examinations, these known devices and techniques are generally not suitable for "routine" office examinations, in which factors such as ease of use, speed of use, comfort, cost, accuracy and reproducibility are of major consideration. Indeed, existing devices are often heavy, unwieldy, cumbersome and require that the devices be affixed to the subject using pins, screws, bolts, brackets, staples and the like.
Several methods have been proposed to find the relative position of a scan "slice" by using anatomical landmarks. In these methods, the size and position of predetermined anatomical structures, such as the lateral end of the internal auditory canal, are used as reference points to help locate and compare lesions and other features of interest. See, e.g., Tan, K. K. et al., in J. Neurosurg. (1993) 79:296-303. A problem with this technique, however, is that the image resolution in the scan slice direction (i.e., the z-direction) is poor compared with the resolution in the scan in-plane (x-y direction). Because of this poor resolution, it is difficult to make precise positional determinations. Moreover, these methods also require a degree of anatomical knowledge which may strain the capabilities of the average MR technologist.
As an alternative, the art has developed devices, such as frames and "halos," to facilitate positioning for stereotactic surgery. These devices are rigidly affixed to the patient being imaged and to an imager platform and provide reference points or lines to facilitate the determination of the orientation of the patient's head. See, e.g., U.S. Pat. No. 4,341,220, which discloses a stereotactic surgical frame with fiducial plates that surround the patient's head in the fashion of a boxer's headgear and which provides several non-collinear fiducial points in cross-sectional scans. Most stereotactic frames are fixed to the patient's skull directly, usually by bolts or screws, as noted previously. Clearly such methods are not suitable for "routine" office examinations.
So-called non-invasive, stereotactic devices have also been described. The Gill-Thomas stereotactic frame, which is based on the Brown-Roberts-Wells neurosurgical frame, was designed to be used for a series of stereotactic radiotherapeutic operations. See, Graham et al., in Radiotherapy and Oncology, (1991) 21:60-62. This device requires that it be affixed to the patient by a block, tailored for individual patients.
Another device, designed by Laitinen et al., is fixed to the patient by means of a nasion support and two ear plugs. See, e.g., Laitinen et al., in Surg. Neurol. (1985) 23:559-566 and U.S. Pat. No. 4,617,925. However, this device is then affixed to the imaging couch or table. Hence, this device is able to permit reproducible scans only by relying on the fixed position of the patient against the couch or table with respect to the machine coordinate system. Again, affixing the patient to the machine may makes the patient uncomfortable during the scan. See, also, U.S. Pat. No. 5,330,485, disclosing a cerebral instrument guide frame that rests on the bridge of the nose (i.e., about the nasion) and which contains plugs for insertion into the external ear canals.
Stereotactic devices are typically fashioned from precision aluminum alloy and are very expensive for all except non-routine use. Moreover, it is usually cumbersome and time consuming to affix these devices to the patient, adding to their unsuitability for routine examinations.
The state of the art suggests that stereotactic devices be equipped with radiographic markers that are visible in scans of a patient's head. For example, U.S. Pat. No. 4,923,459 discloses a stereotactic frame that also includes radio-opaque rods arranged in the configuration of the letter "N" to facilitate localization of a surgical target. U.S. Pat. No. 4,608,977 discloses a helmet-like, stereotactic frame that includes such N-shaped "localizing" rod to facilitate the determination of the location of a CT scan cross-section. Likewise, U.S. Pat. No. 4,638,798 discloses a halo-like stereotactic frame that has a ring with a plurality of pins of differing lengths extending therefrom. The relative location of a scan can be determined from the relative location of the ends of the pins.
Though such devices can be used to determine the location of a head in x-y space, and to determine the relative location of each imaging "slice," they do not permit the position of a head to be fully determined, e.g., as where the head is tilted in the imaging plane.
In addition to the limitations described above, the prior techniques are not generally suitable for direct alignment of images obtained from different imaging modalities. That is, to permit the direct comparison of images obtained from different imaging modalities, say MR and CT, the patient must be re-aligned precisely with respect to the two machine coordinate systems. Alternatively, a correction can be made using image processing techniques after a second or subsequent scan has been taken. However, image processing has the drawback in that the resolution of the processed image is dependent on the quality of the scan data set. It would be desirable to alter a scan in real time such that scans from different modalities can be compared directly without the need for image processing.
In each of the known devices and methods, the anatomical coordinates of the patient are fixed in relation to a reference coordinate system, that is the machine's coordinate system. Thereafter, the machine's coordinate system is used as the reference coordinate system for each subsequent scan. Because of the difficulty in reproducing the machine coordinate system or because different machines are invariably associated with different, incompatible machine coordinate systems, it has not before been possible to relate directly scans from different imaging modalities. Moreover, it is not always possible to align directly scans from the same imaging modality (e.g., MR imagers) when comparing images obtained from machines made by different manufacturers.
It would thus be desirable to have a device and method whereby the reference coordinate system is independent of the machine or imaging modality. It would be desirable, moreover, to use a reference coordinate system "personal" to the patient as the reference coordinate system and, where possible, have the machine's coordinate system fixed or adjusted relative to that of the patient to provide for scans that are reproducible, compatible and superimposable in the same or different imaging modalities. A system that enables the taking of imaging scans under such a patient reference or "personal" coordinate system would be of great utility and would be deemed a significant advancement in the art.
In view of the foregoing, it is an object of the invention to provide improved devices and methods of non-invasive, repetitive, radiographic examination of a subject, particularly of the subject's head.
A further object of the invention is to provide such devices and methods that are readily amenable for use in "routine" examinations, as well as for surgical planning and follow-up.
A still further object of the invention seeks to provide improved devices and methods of stereotaxis (both invasive and non-invasive), which are low-cost, easy to use, comfortable and which provide accurate and reproducible results.
Yet another object of the invention relates to improving methods and apparatuses that can determine fully the position of a head and a scan plane, including when the head is tilted in the scan plane.
Other objects of the invention include providing a way or means for comparing directly scans taken by the same or different imaging modalities and providing a method for the reproducible placement of external markers, e.g., electrodes, on a patient.